Backups - no disk space??? Home and Business 2016

When trying to backup I get a message telling me that the backup disk is out of space and to insert a new disk. The problem is that there is plenty of space on the disk or any other device eg USB stick, that I care to use. What causes this problem and how can it be fixed?

My main backup drive has 1.25TB free of 1.3TB, so that is
not the problem. My USB has 23.8GB free of 58.9GB. So they are not the problem.
I have tried to back up on many other drives and I get the same message “A
backup error occurred. Please check your files were backed up to the selected
location”. The same as it has been for months now.

I realise that it automatically backs up on my computer and
I can cut & paste from there, however this does not seen to work from my
laptop to a USB.

Hmmm yes sorry I can't help, just thought I would toss the obvious out there and confirm ALL drives have enough space. Sometimes a random drive is picked for a temporary working area, and that might not have had enough space causing it to fail. Seems that's not the case.